1. Field of the Invention
The present invention generally relates to power management in a carrier sense multiple access with collision avoidance (CSMA/CA), and more specifically, relates to asynchronous power management thereof.
2. Description of Related Art
Current wireless data communication systems have been developed on the assumption that the communication systems use their own frequency bands. However, the 2.4 GHz Industrial, Scientific and Medical (ISM) band is shared by systems including wireless local area networks (LANs), Bluetooth devices, home radio frequency (RF) devices, medical equipment, and microwave devices and the like. As a great number of systems use the limited frequency band, collisions are unavoidable in transmitting and receiving modulation frequencies.
In order to avoid interference or data collisions between communication systems, a carrier sense multiple access with collision avoidance (CSMA/CA) scheme is utilized. The CSMA/CA scheme sends a signal indicating the presence of data to be transmitted before a sending end transmits the data, and transmits the data only upon receiving a response to the signal from a receiving end. Accordingly, it is possible to prevent and avoid data collisions with other data transmitted by other sending ends.
According to the CSMA/CA scheme, each device carries out power management to save the power consumption. Each device repeatedly enters a sleep mode and a wake-up mode to reduce the power consumption. A device in the sleep mode suspends data transmission and/or reception, and a device in the wake-up mode restarts to transmit and/or receive data to and from other devices.
The power management includes synchronous power management and asynchronous power management, which are explained in that order.
The synchronous power management synchronizes with all devices in a network. Simultaneously, the synchronized devices maintain the wake-up mode in a certain period. A device storing data to be transmitted to another device in the wake-up mode sends a signal indicating the presence of such data. Other devices receiving the signal maintain the wake-up mode.
A device not receiving the signal indicating the presence of the data to be transmitted to another device or a device having no data to be transmitted maintains the sleep mode. The synchronous power management has a disadvantage of higher power consumption since all the devices repeatedly and uniformly enter the sleep mode or the wake-up mode. The asynchronous power management is presented to address this problem.
According to the asynchronous power management, the devices in the network independently enter the sleep mode and the wake-up mode. That is, the devices each adjust their states into the sleep mode or the wake-up mode depending on their power condition. A device storing a large amount of power sets the wake-up mode longer than the sleep mode, and a device storing a small amount of power sets the sleep mode longer than the wake-up mode. The data transmission and reception of the asynchronous power management are the same as those of the synchronous power management.
FIG. 1 depicts a first device 100 which receives data and a second device 102 which transmits the data according to the asynchronous power management. Referring to FIG. 1, drawbacks of the conventional asynchronous power management are described.
The first device 100 is in the sleep mode and the second device 102 is in the wake-up mode as shown in FIG. 1. When the second device 102 has data to be transmitted to the first device 100, the second device 102 needs to inform the first device 100 of the presence of the data to be transmitted prior to the data transmission. Hereinafter, a signal indicating the presence of the data to be transmitted is referred to as a request-to-send (RTS) message.
The second device 102 sends the RTS message to the first device 100. The first device 100 being in the sleep mode cannot receive the RTS message, and also cannot send a response message to the RTS message. Thus, the second device 102 recognizes that the first device 100 does not accept the data transmission, and repeats transmission of the RTS message to the first device 100. As a result, the power consumed by the second device 102 considerably increases.